BPL requires the transfer of high frequency (HF) signals, from communication devices operating at ground potential, to medium voltage (4-35 kV) power lines, and vice versa. Couplers are used for such transfers, and these couplers must accurately inject and extract tiny HF signals while being unaffected by the much larger currents and voltages (having low frequencies on the order of 50 or 60 Hz) that normally exist on medium voltage (MV) power lines. In addition, these couplers must be designed to withstand the transient voltages and currents that occasionally occur on MV lines as a result of switching operations, lightning strikes, electrical faults and other such phenomena. Furthermore, these couplers must be designed to fail in a manner that does not create a safety hazard for the general public, which normally is at or near ground potential. If a coupler fails to meet these essential criteria, then the coupler not only fails to serve any purpose, but can actually cause damage and danger.